I’ve just had an email from a customer about an order through one of our osCommerce run shops saying they had placed an order but when they had gone through to PayPal, they received a message saying

“ERROR DETECTED: This invoice has already been paid. For more information, please contact the merchant.”

I checked PayPal and there was no transaction but when checking in osCommerce the customer had placed an order which appeared to be paid for, After a bit of checking I found out what was causing the problem and the fix as well!

It appears that when using the PayPal IPN, it requires a unique Invoice Number for each transaction which isn’t a problem most of the time. We’ve had a problem because we’re using the same PayPal account (email address) for several shops. Unfortunately since we’re using seperate installations of osCommerce and therefore seperate databases for each, there’s sometimes a overlap of invoice numbers.

Luckily we aren’t the first and you can find the solution on this osCommerce Forums Thread

This entry was posted on Wednesday, July 4th, 2007 at 7:05 pm and is filed under Web Development. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.